In recent years, in security equipment used in large-scale facilities such as airports and plants and lifeline-related facilities such as power plants and water service facilities and in traffic information assist systems such as ITSs (intelligent transport systems), many systems have been put on the market which track a target using an ITV (industrial television) camera or the like to thereby continuously monitor the target and acquire detailed information. Assuming not only ground-installation-type uses but also uses with platforms of automobiles, ships, airplanes, etc., these systems have compact, earthquake-resistant structures and are capable of suppressing disturbance such as vibration or a jolt. Furthermore, it is now important that the turning speed be sufficiently high and the system be capable of being directed to a target in a short time so that plural targets can be traced sequentially.
To track a target that moves in a wide range, such mobile object image tracking systems in many cases employ a gimbal structure having at least two rotation axes. In this case, image tracking control is performed so that a feature point of a mobile object in an image taken by a camera that is provided in a gimbal movable unit is aligned with the center of the camera. An example of such configuration is disclosed in JP-A-8-321984.
In the above technique, since the camera is provided in the gimbal movable unit, miniaturization of the system is difficult and the load inertia is large. As a result, it is difficult to track a mobile object that moves at high speed. Furthermore, high-speed tracking requires high motor performance.
One solution for solving the above problems is to dispose the camera outside, rather than inside, the movable unit. Examples of such solution are disclosed in JP-A-2001-133854 (counterpart U.S. publication is: U.S. Pat. No. 6,927,905 B1) and in JP-A-2002-314487.
However, the devices disclosed in the documents JP-A-2001-133854 and JP-A-2002-314487 are associated with a problem that an image taken by the camera provided outside the movable unit is rotated depending on the directions of a gimbal front face and the light incident surface of a camera optical system that includes mirrors and is rotated about the optical axis.